There are no standards. Drop shadows should just be realistic. No huge depth for a piece of paper laying on the base, and not really shallow shadow for something that's hugely bulky.
I happen to use Traci Murphy's drop shadow actions (avaialble at funkyplaygrounddesigns) as a starting point, and adjust them as I see need on a particular object. I often use ones that aren't named for that particular action because I like how it makes the element look. I like to use her actions because it saves me a lot of time.
But there are great tutorials out there for learning how to do drop shadows on your own, depending on what program you use.